1. Field
The aspects of the present disclosure relate generally to high intensity discharge (“HID”) light sources and in particular to bi-power electromagnetic ballasts for driving HID lamps.
2. Description of Related Art
A high intensity discharge or HID lamp is type of electric lighting device capable of producing a high level of light for its physical size by means of an electrical discharge. A controlled high intensity arc is maintained between two electrodes disposed within a glass or ceramic tube which is filled with gas and metal vapors. In general, HID lamps are favored for their long life, high light output, small size, and improved electrical efficiency as compared to fluorescent and incandescent lighting technologies. HID lamps are typically named by the type of gas and metal contained within the arc tube. Some of the more popular HID lamp types are high pressure sodium (HPS), quartz metal halide (QMH), and ceramic metal halide (CMH).
HID lamps, like fluorescent lamps, require a ballast circuit to provide the proper starting voltage for the lamp and limit operating current once the lamp is ignited. A ballast circuit or ballast is an electric circuit that limits the amount of current flowing through the lamp allowing long lamp life and efficient operation. HID lamps exhibit a negative impedance characteristic, which means that the lamp draws more current than is required for it to operate. Without a ballast circuit, running the lamp in this negative impedance condition would cause the lamp to self-destruct in a very short period of time. Typically, a HID ballast (sometimes with the addition of a capacitor and igniter) serves to start and operate the lamp in a controlled manner.
HID lamps can be driven from modern electronic ballasts which achieve power regulation, controllability, and are energy efficient. However, electronic ballasts are complex and costly, making them less desirable for some applications. Electromagnetic ballasts provide a low cost solution that is often more desirable than the high cost electronic ballasts. Electromagnetic ballasts use magnetic components to start and regulate the operation of a lamp and limit lamp current using inductors. Inductors cause a phase shift between the supply voltage and the current resulting in a reduced power factor. Often times, a capacitor is included in the ballast circuit to increase the power factor and improve overall efficiency of the lighting system.
There are additional factors that need to be considered when using HID lighting systems. HID lamps do not achieve their full light output immediately after starting. They require a warm-up period, which for certain types of metal halide lamps can be as long as 15 to 20 minutes. After a HID lamp has been on for a period of time and then extinguished, it cannot be immediately turned back on. The arc tube must have a chance to cool down or the lamp will not restart. This period of time is called the restrike time. Restrike times for HID lamps can be quite long. For example, a probe start type QMH lamp can have a restrike time of 10 to 20 minutes, while a HPS lamp may require 1 to 3 minutes before the lamp can be re-ignited.
In certain applications it is desirable to reduce the light output of HID lamps. For example, a HID lamp may be dimmed when the area they are lighting is unoccupied, or when full light level output is not desired. In these applications HID ballasts have been designed to support a dimmed mode of operation where the lamp power is dropped by as much as 50%. This can translate to significant energy cost savings in many HID lighting applications.
There are two general classes of HID dimming systems. In bi-level dimming, also known as bi-power dimming, HID lamps are run at two distinct power levels. A reduced power or dimmed power is used when less light is desired and full power is used when full lamp brightness is desired. Bi-level dimming systems are sometimes designed to occasionally raise the power level to full brightness during prolonged periods of dimmed operation to improve lamp life. The other common class of dimming system is called “continuous” dimming and allows users to select a desired wattage from a continuous range of wattage values thereby providing users with complete light control. Continuous dimming ballasts are more complex and costly than bi-level ballasts making them less desirable than bi-level systems in many lighting applications.
When HPS lamps and some CMH lamps, such as those used in street lighting, are used with bi-level dimming electromagnetic ballasts, they may drop out when switched to dimmed wattage operation. This failure depends on lamp voltage where higher voltage lamps are more susceptible to drop out due to a transient effect of temporary lamp voltage rise. Dimming of bi-level ballasts is typically achieved by lowering the lamp power in a single step after which the lamp voltage may rise for a short period of time. As lamp voltage rises to near the open circuit supply voltage, the lamp may drop out or extinguish thereby requiring a long cool down period before re-strike can occur.
Accordingly, it would be desirable to provide bi-power electromagnetic ballasts that address at least some of the problems identified above.